The applicant of this application had disclosed a linkage mechanism in U.S. 2017/0243704 A1. The linkage mechanism includes a left wing having two left arms, two left supporting rods that are respectively formed on the left arms, and two left sliding rods that are respectively formed on the left arms, a right wing having two right arms, two right supporting rods that are respectively formed on the right arms, and two right connecting rods that are respectively formed on the right arms, and a hinge unit having two left hinge pins, two right hinge pins, two left hinge holes and two right holes. Each left hinge hole is formed in a respective left arm and is engaged with a respective right hinge pin. Each right hinge hole is formed in a respective right arm and is engaged with a respective left hinge pin.